The field of the manufacture of impact modified plastics is relatively old and the current industrial processes for their manufacture are fairly well known. According to conventional technology, typically a solution of rubber, typically comprising 1 to about 20, preferably from 3 to 12 weight %, most preferably 4 to 10 weight % of rubber dissolved in one or more monomers is polymerized in a first stage reactor under mechanical agitation. The first stage of the polymerization may be carried out in a batch process or preferably may be carried out in a continuous process. Continuous process configurations would include continuous stirred tank reactors, continuous stirred loop reactors or stirred plug flow reactors.
In the production of high impact polystyrene in a batch process or in a stirred plug flow reactor, initially the rubber-like composite phase is the continuous phase and the resin phase (monomer/resulting polymer phase) is dispersed. Typically, in conventional processes, as the polymerization proceeds in time with a batch reactor or in space with a stirred plug flow reactor at some point between 5 and 20% conversion the system undergoes particulation by phase inversion under the application of a shear field generated by mechanical agitation. That is, the rubber-like composite phase becomes the dispersed phase and the resin phase becomes the continuous phase. This does not happen instantaneously but occurs over a period of time or space, typically from 10 to 50 minutes or reactor space which produces 2 to 8% conversion. That is, the rubber-like composite phase and resin phase become co-continuous for a period of time before the particulation process is complete.
The ternary phase diagram of the styrene-polystyrene-polybutadiene system has been well studied and is well known. For example, the phase diagram and what happens during the polymerization of high impact polystyrene is discussed in Kirk-Othmer Encyclopaedia of Chemical Technology, published in 1983, Volume 21, pages 823 through 826.
In the production of high impact polystyrene in a continuous stirred tank reactor or a continuous stirred loop reactor the rubber phase is particulated by the mechanism of dispersion. That is, the rubber or rubber-like composite phase is quickly dispersed as droplets in a continuous stirred tank or continuous stirred loop reactor that is operated with a continuous resin phase.
Applicants have been able to locate little art relating to processes for making resins that contain a large particle size dispersed phase. Generally in the art the conventional wisdom is that the upper particle size is limited by the need to provide sufficient mixing in the reactors to meet heat transfer requirements and to prevent reactor fouling. Therefore the maximum attainable particle size is system dependent.
Canadian Patent 1,160,791, issued Jan. 17, 1984 to the Dow Chemical Company discloses a process to polymerize a high impact polystyrene (HIPS) having a dispersed phase (rubber phase) particle size of about 3.8 micrometers by including a major amount of a C.sub.6-10 aliphatic hydrocarbon in the syrup. The present invention does not contemplate inclusion of a major amount of a C.sub.6-10 aliphatic hydrocarbon in the syrup. Thus Applicants have eliminated an essential feature of the prior art.
U.S. Pat. No. 4,187,261, issued Feb. 5, 1980, assigned to Standard Oil Company teaches a similar approach to preparing a HIPS having a large particle size. According to the reference the polymerization is carried out at a relatively low conversion so there is excess monomer in the syrup when it is devolatilized. In effect this may act as a hydrocarbon solvent/diluent. The particle size is up to 9 micrometers. Again the process of the invention is directed to a chemical method rather than a mechanical method of producing such a large particle size distribution.
South African patent publication 811001 assigned to BASF discloses a bulk suspension process using high amounts of rubber (28%) in a styrenic phase. The dispersed phase particles have a size of about 4.5 micrometers. The present invention does not contemplate such a high content of rubber or the use of a suspension process.
U.S. Pat. No. 4,388,447 issued Jun. 14, 1983, assigned to Mitsui Toatsu Chemicals, Inc. teaches a process for producing rubber modified styrenic resins in which the rubbery phase including rubber-like polymer is transformed into dispersed particles in a stirred tank reactor having both a helical blade agitator mounted in a draft tube and an auxiliary agitator. Comparative example 3 discloses the use of the helical blade agitator in a draft tube without the use of an auxiliary agitator. The present invention does not contemplate the presence of an auxiliary agitator nor does it contemplate the presence of a draft tube. That is, in accordance with the present invention the auger agitator is used without, or in the absence of, a draft tube.
U.S. Pat. No. 4,587,294 issued May 6, 1986, assigned to Mitsui Toatsu Chemicals, Inc. and Toyo Engineering Corporation teaches a process for the manufacture of ABS in which the reactor in which phase inversion takes place has a screw type agitator mounted in a draft tube in conjunction with an auxiliary agitator at the bottom. As noted above, the present invention does not contemplate the presence of an auxiliary agitator nor does it contemplate the presence of a draft tube.
The present invention seeks to provide a mechanical means to prepare a syrup having a volume average particle size distribution of greater than 4, preferably greater than 6, most preferably not less than 8 micrometers.